1. Field of the Invention
This invention relates, in general, to the field of semiconductor manufacture. In particular, it relates to monitoring the presence of a semiconductor wafer during post chemical mechanical polishing (CMP) processing.
2. Description of Related Art
Fabrication of semiconductor integrated circuits (IC) is a complicated multi-step process for creating microscopic structures with various electrical properties to form a connected set of devices. As the level of integration of ICs increases, the devices become smaller and more densely packed, requiring more levels of photolithography and more processing steps. As more layers are built up on the silicon wafer, problems caused by surface non-planarity become increasingly severe and can impact yield and chip performance. During the fabrication process, it may become necessary to remove excess material in a process referred to as planarization.
A common technique used to planarize the surface of a silicon wafer is chemical mechanical polishing (CMP). CMP involves the use of a polishing pad affixed to a circular polishing table and a holder to hold the wafer face down against the rotating pad. A slurry containing abrasive and chemical additives are dispensed onto the polishing pad.
The wafer and polishing pad rotate relative to each other. The rotating action along with the abrasive and chemical additives of the slurry results in a polishing action that removes material from the surface of the wafer. Protrusions on the surface erode more efficiently than recessed areas leading to a flattening or planarization of the wafer surface. Following CMP, the wafer must be cleaned of any CMP and slurry residue. Any residue remaining on the wafer can cause shorts in the IC devices.
The wafers are loaded into a wet processing station such as that shown in FIG. 1, and submerged in a water bath. The processing station contains a wafer transporter having a track which moves the wafers by means of a water jet pushing the wafers along the track. The wafers are then pushed into a cassette-like holder having horizontal slots for each wafer. The holder is motorized moving vertically to receive the wafer in the slot as it is being shot off the wafer transporter.
One or more sensors are used to track the presence of the wafers along the wafer transporter and prior to being inserted into the slots of the wafer holder.
Prior art methods employ sensors to track the presence of the wafers which utilize interruption of a signal path, for example, optical sensors using fiber optics. However, the optical sensors have proven unreliable due to film build-up, wafer color/hue variations, light refraction, sensing distance, bubble interference in water tracks, and other wet environment associated problems. These "false" sense events result in wafer breakage due to the inability of the sensor to correctly detect the presence of the wafer. The wafer breaks apart as the timing between the transport track and the holder are not in sync. Lost tool production in stopping the process adds to the already high cost of semiconductor wafer fabrication.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a non-optical sensor which overcomes the problems of false sensing.
It is another object of the present invention to provide a sensor which is sustainable for use in a wet environment without regard to slurry contamination or CMP residue.
A further object of the invention is to provide a method of sensing a semiconductor wafer in a CMP processing station.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.